Metal Gear Solid V: The Phantom Pain companion app is now available for download on Android and iTunes. This companion app is non-canonical and has no story value. One of its major function is that it allows Metal Gear Solid V: The Phantom Pain players to control iDroid, start screen in-game via their mobile device to view the map. Additionally, it will also allow player to listen to music as well as briefing tapes and to check out the map.

Here is an interesting Metal Gear Solid V: The Phantom Pain guide showcasing how to effectively use iDROID.

Use data device: Touch pad button (right).

Allows you to use the Mother Base-developed "iDROID" data device. You can change it with L1/R1. Please note: Metal Gear Solid V: The Phantom Pain gameplay won't pause while you use the iDROID.

Aerial Command Center

Connecting the iDROID to your helicopter's onboard computer turns it into your Aerial Command Center. When in the Aerial Command Center, several menus are added to the iDROID, including the ability to move to another area of operations or Mother Base, plus various customization options.

Cassette Tapes:

You can access this in [CASSETTE TAPES] under the iDROID's [MISSIONS] tab

Cassette tapes obtained during gameplay can be played by going to [CASSETTE TAPES] in the iDROID's [MISSIONS] tab. In addition, when the cassette tape icon is showing while you are in the Aerial Command Center (your helicopter), press the "Triangle" button to switch to the cassette tapes screen.

You can check out the screenshot below showcasing Cassette Tape icon

Here is the list of Cassette Tape Types in Metal Gear Solid V: The Phantom Pain:

Info Tapes: These contain information pertaining to the story.
Music Tapes: Various music recordings that can also be set as your helicopter's arrival music.
Acquired Tapes: Some tapes enable you to influence your surroundings under specific conditions if you turn on an external speaker from the control panel (R2)

Briefing Tapes:

Go to [MISSION LIST] under the iDROID's [MISSIONS] tab, highlight a mission, and press the "Triangle" button to play a tape with information pertaining to that mission.
Use these tapes as a reference when selecting a mission.
These tapes can also be played during a mission by going to the iDROID's [MISSIONS] tab.